CSFB is specified today in the 3rd Generation Partnership Project (3GPP) Technical Specification (TS) 23.272 (see, e.g., V9.4.0 of June 2010). In brief, CSFB permits in an Evolved Packet System (EPS) the provisioning of voice and other CS domain services (such as services pertaining to Unstructured Supplementary Service Data, or USSD) by re-use of CS infrastructure when the UE is served by the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). A CSFB-enabled UE, connected to E-UTRAN, may thus use 2nd or 3rd Generation (2G or 3G) technologies such as GSM Edge RAN (GERAN) or UTRAN to connect to the CS domain.
FIG. 1 illustrates an exemplary scenario in which a CSFB-enabled UE connected to E-UTRAN (via the LTE-Uu interface) uses GERAN (via the Um interface) or UTRAN (via the Uu interface) to connect to the CS domain in a CSFB situation (see section 4 of 3GPP TS 23.272). CSFB signaling is realized via the SGs interface between a Mobility Management Entity (MME) associated with the UE and a Mobile Switching Centre Server (MSC-S). The MSC-S has to be CSFB-enabled, which basically means that the MSC-S will have to be capable of maintaining SGs associations towards the MME for EPS attached UEs. A Serving GPRS Support Node (SGSN) is coupled via the S3 interface to the MME and via the Gs interface to the MSC-S.
The SGs interface to the MSC-S is used for mobility management and paging procedures between EPS and the CS domain. CSFB is triggered by paging in E-UTRAN and leads to a Location Update with respect to GERAN or UTRAN (see FIG. 1).
3GPP TS 23.018 (see, e.g., V9.2.1 of October 2010) describes basic call handling mechanisms. An exemplary call handling mechanism for a terminating call to a so-called B subscriber is shown in FIG. 2 (see section 4.2 of 3GPP TS 23.018) for a roaming UE (also called Mobile Station, or MS). The roaming UE is located in a Visited Public Land Mobile Network (VPLMNB).
As illustrated in FIG. 2, when a Gateway MSC of the called B subscriber (GMSCB, or simply GMSC) receives an ISDN User Part (ISUP) Initial Address Message (IAM) for a UE, it requests routeing information from the associated Home Location Register of the B subscriber (HLRB, or simply HLR). To this end, a Send Routing Info (SRI) message is sent to the HLR. Next, the HLR requests a roaming number (i.e., a Mobile Station Roaming Number, MSRN) from the Visitor Location Register currently associated with the B subscriber (VLRB, or simply VLR). In this regard, a Provide Roaming Number (PRN) message is transmitted to the VLR. In a further step the VLR returns the MSRN in a PRN Ack message to the HLR, which forwards the MSRN to the GMSC in a SRI Ack message. The GMSC uses the MSRN to construct an ISUP IAM, which is sent to the Visited MSC of the B subscriber (VMSCB, or simply VMSC).
Upon receipt of the JAM, the VMSC requests information to handle the incoming call from its associated VLR. If the VLR determines that the incoming call is allowed, it requests the VMSC to page the UE. In a next step, the VMSC pages the UE via a Base Station Subsystem associated with the B subscriber (BSSB) using radio interface signaling. In this context, the VMSC starts a local paging timer. When the UE responds, the VMSC informs the VLR in a Page Ack message thereof and stops the paging timer. Next, the VLR instructs the VMSC to connect the call in a Complete Call message, and the VMSC finally establishes a traffic channel to the UE.
The mechanism for handling a terminating call illustrated in FIG. 2 needs to be modified in certain CSFB scenarios. Specifically, CSFB is only available in case E-UTRAN coverage (as defined by Tracking Areas, or TAs) is overlapped by either GERAN or UTRAN coverage (as defined by, for example, Location Areas, or LAs). A general problem results from the fact that there exists no 1:1 mapping between TAs and LAs.
As shown in FIG. 3, for a terminating call the lacking congruency between TAs and LAs may have the consequence that the UE, when falling back from E-UTRAN to GERAN or UTRAN, may land in a LA that is not controlled by the “old” MSC-S towards which the SGs interface association for the UE has been established by the MME. In such a case the “old” MSC-S will not be able to terminate the call. For this reason, a so-called Roaming Retry (RR) procedure is defined in section 5.2 of 3GPP TS 23.018 to allow that call termination is tried again by the GMSC towards the “new” MSC-S controlling the cell the UE is presently camping on.
The RR procedure suggested in section 5.2 of 3GPP TS 23.018 results in the requirement that all GMSCs have to be upgraded for RR support. Such an upgrade may be difficult for network operators having MSC-S and GMSC from different vendors because in practice every MSC-S may as well play the role of a GMSC, and hence all vendors have to provide RR support and all MSC-Ss have to be upgraded. For large countries such as China hundreds of MSC-Ss thus require an update prior to launching CSFB. The fact that the GMSC is located in the home country of the subscriber while, when roaming, the serving MSC-S is located in a visited network further complicates the situation. In such a case RR support has to be coordinated among different network operators, often across international borders.
All these considerations put the current GSM Association strategy at risk, which is to mandate CSFB as the early roaming solution for Long Term Evolution (LTE) networks.
Summary
Therefore, a technique for efficiently controlling set up of a terminating call in a CSFB situation is required that avoids one or more of the drawbacks of the existing RR scenario summarized above.
According to a first aspect, a method of controlling set up of a terminating call to a UE in a CSFB situation involving a first MSC paging the UE and a second MSC were the UE is performing a Location Update is provided, wherein the method is performed by the first MSC and comprises receiving, from an HLR, a PRN message and forwarding information received in the PRN message towards the second MSC. It will be appreciated that the term “MSC” also encompasses an MSC-S and a combination of an MSC-S/MSC and an associated VLR.
The first MSC may at least temporarily store the PRN message or information received therein (e.g., subscriber data) locally. The storing may be performed for forwarding and/or any other purposes. The information may be kept (e.g., in a VLR associated with the first MSC) until expiry of a timer or receipt of a notification message from the second MSC.
The method may further comprise receiving a notification message from the second MSC. In one implementation, the notification message is received during a Location Update procedure. The notification message may trigger an enhanced RR procedure to the first MSC. The enhanced RR procedure may include (or involve) the forwarding of the information received in the PRN message towards the second MSC.
The information received in the PRN message that is forwarded towards the second MSC may include one or more of the conventional MAP_PROVIDE_ROAMING_NUMBER parameters (see, e.g., section 10.2 of 3GPP TS 29.002). As an example, the information received in the PRN message and forwarded to the second MSC may include the MSC number of the first MSC. The MSC number of the first MSC may be utilized by the second MSC for the purpose of sending a PRN reply (e.g., an acknowledgement) to the first MSC. The method performed by the first MSC may further comprise receiving, from a GMSC, a call set up request messages and forwarding the call set up request message towards the second MSC. In one implementation, the UE is paged by the first MSC in response to receipt of the call set up request message.
The first MSC may control a paging timer. The paging timer may be started, and it may be stopped again by the first MSC in response to receipt of a Cancel Location message from the HLR.
According to a first variant, the first MSC has an SGs interface association with an MME for the UE. According to a second variant that may be combined with the first variant, the second MSC has no SGs interface association with the MME for the UE.
The method may be performed in the context of the UE camping on at least one of a 2G cell and 3G cell not served by the first MSC. In the present CSFB context, the UE may camp on that cell after CSFB to 2G/3G has happened.
In one implementation, the second MSC is different from the first MSC. As an example, the first MSC may be the “old” MSC where the UE was registered before the CSFB situation, while the second MSC is the “new” MSC where the UE is performing Location Update.
According to another aspect, a method of handling set up of a terminating call to a UE in a CSFB situation involving a first MSC paging the UE and a second MSC where the UE is performing a location update is provided, wherein the method is performed by the second MSC and comprises receiving, from the first MSC, PRN information, receiving also from the first MSC, a call set up request message, and handling call set up to the UE in accordance with the PRN information.
The method performed by the second MSC may further comprise receiving a Location Update message from the UE. The Location Update message may include a Circuit Switched Mobile Terminating (CSMT) flag. In one implementation, the CSMT flag is interpreted by the second MSC to indicate to the second MSC that the Location Update is due to CSFB.
According to a further variant, the method performed by the second MSC also comprises triggering an enhanced RR procedure to the first MSC. The triggering may occur responsive to the presence of the CSMT flag in the Location Update message. Additionally, or as an alternative, triggering the enhanced RR procedure to the first MSC may comprise sending a notification message towards the first MSC. This notification message may be sent during a Location Update procedure.
The technique presented herein may be realized in the form of software, in the form of hardware, or using a combined software/hardware approach. As regards a software aspect, a computer program product comprising program code portions for performing the steps presented herein when the computer program product is executed on a computing device is provided. The computer program product may be stored on a computer-readable recording medium such as a memory chip, a CD-ROM, a hard disk, and so on. Moreover, the computer program product may be provided for download via a network connection onto such a recording medium.
According to a still further aspect, an MSC for controlling set up of a terminating call to a UE in a CSFB situation involving another MSC adapted to perform a Location Update procedure with the UE is provided. The MSC is adapted to page the UE and comprises a receiving component adapted to receive, from an HLR, a PRN message, and a forwarding component adapted to forward information received in the PRN message towards the other MSC.
With regard to another aspect, an MSC for handling set up of a terminating call to a UE in a CSFB situation involving another MSC paging the UE is provided. The MSC is adapted to perform a Location Update procedure with the UE and comprises a receiving component adapted to receive, from the other MSC, PRN information, and further adapted to receive, from the other MSC, a call set up request message, and a handling component adapted to handle call set up to the UE in accordance with the PRN information.